Editor’s note: This two-part article is one of a series of Best Practice Guides for laboratories, produced by Laboratories for the 21st Century (“Labs21”), a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy. Geared toward architects, engineers, and facility managers, these guides provide information about technologies and practices to use in designing, constructing, and operating safe, sustainable high-performance laboratories. For more information about these free resources, see: www.Labs21century.gov/toolkit/bp_guide.htm. The Labs21 website also provides full information about the agency’s upcoming annual conference, to be held in North Charleston, S.C., Oct. 2-4. The co-meeting is sponsored by I2SL, the International Institute for Sustainable Laboratories.
The first article,which was published in June, covered load variation and energy use in labs. The second part discusses specific strategies for minimizing reheat.
As the simulation analysis printed in the June issue shows, equipment load variation in laboratories can increase energy use in laboratories that have systems with reheat. The magnitude of this increase varies with location, ventilation rate, and degree of variation. We will now look at strategies for minimizing reheat.
Carefully assess equipment loads, and use profiles during design. The first step in minimizing reheat energy use is to properly assess it during the design process. Often, HVAC designers assume uniform loads across the labs and do not account for the variation that inevitably occurs. Energy simulations used during design should model the reheat energy use caused by load variation. The simulations should model actual load in each zone instead of an average.
The Labs21 Modeling Guidelines [Labs21 2004], which are designed to be used in conjunction with the ASHRAE 90.1 standard [ASHRAE 2001], specify a standardized approach to incorporating load variation into the simulation models used for compliance and benchmarking.
Fig. 1. Alternative HVAC systems that minimize reheat energy use caused by load variation between zones. Diagrams: Labs21.
Consider alternative HVAC systems. There are several different HVAC system alternatives that can mitigate reheat energy use, as shown in Fig. 1 (above) [Morehead 2003]. They all involve separating the thermal and ventilation systems, as follows:
• Dual-duct with terminal heating (DDTH). This system consists of two separate variable volume supply air streams: one with tempered air, and one with cold air. Labs that require more cooling will draw more air from the cold air stream while others will draw primarily from the tempered air stream.
• Zone cooling and heating coils (ZC). This system has a single tempered supply air stream, with the primary cooling and heating provided by zone heating and cooling coils. The temperature of the tempered air stream will be adjusted to minimize or eliminate the requirement for any zone reheat. This system has been installed in two laboratory buildings at Lawrence Berkeley National Laboratory.
• Ventilation air with local fan coils (FC). This is similar in principle to the zone cooling and heating coils. The difference is that the heating and cooling occurs with fan coil units rather than coils directly in the ventilation air stream. Note that implementing a fan coil in a space may require coordination with and education of local authorities if there are any prohibitions in the local codes on air re-circulation in a laboratory space. A properly implemented fan-coil system will not mix air between any zones and will have no impact on space pressurization and ventilation rates. While it does not violate the intent of most code regulations, this approach may be unfamiliar and may require educating and gaining the approval of inspectors. This system was a key energy efficiency feature of the Koshland Integrated Natural Science Center at Haverford College, Haverford, Pa. During the summer, no heat is used by the system (the heating supply is shut off); it is therefore a system that literally does not use any reheat. For more information on this building, see the Labs21 case study [Labs21 04].
• Ventilation air with radiant cooling (RC). This system also has a tempered supply air stream for ventilation. Space cooling is provided by radiant panels or chilled beams. Space heating is provided by zone heating coils located in the supply air stream. The major concern with radiant cooling is avoiding condensation by ensuring that the wet bulb temperatures are below dew point. This requires robust and reliable sensors and controls. Chilled beams have been applied in laboratories in Europe (Fig. 2, below) and are beginning to be applied in the U.S. as well. Chilled beams are especially being considered in retrofit applications as a way to provide additional cooling capacity in labs that are already running at maximum airflow capacity.
Fig. 2. Chilled beams have been installed in laboratories in Europe and are beginning to be applied in the U.S. as well. Diagram: TroxUSA.Click to enlarge
Note that these alternatives do not necessarily imply constant volume air supply. They may still require variable air supply to account for variable volume fume hoods or variable general ventilation requirements.
Costs of alternative systems. These systems may have a higher construction cost than the conventional VAV system with reheat. A study by Davis Langdon compared the HVAC construction costs for a base-case VAV reheat with four alternatives: 55° dual duct system, 45° dual duct system, fan coil system, and a radiant panel system. The cost analysis was done on a 150,000-ft2 prototypical laboratory space located in the San Francisco region, based on summer 2005 cost data. Fig. 3 (below) summarizes the key findings:
• The HVAC system construction cost for the base-case VAV system is $72/ft2. The costs for the alternative systems are $78/ft2 for 55° dual-duct, $73/ft2 for 45° dual-duct, $69/ft2 for fan coils, and $84/ft2 for radiant panels.
• The dual-duct system is cost-competitive with the VAV system if it is designed for 45° supply air. A dual-duct system configured for 55° supply air results in increased cost for air handling equipment and air distribution.
• The fan coil system costs less than the VAV. The cost increases for the piping and fan coils are offset by larger cost decreases in air distribution and controls.
• Although the radiant cooling system results in reduced cost for air handling equipment and air distribution, this saving is outweighed by the cost of the radiant panels.
Fig. 3. HVAC cost comparison for a 150,000-ft2 prototypical laboratory space located in the San Francisco region, based on summer 2005 cost data. Data source: Davis Langdon. Diagram: Labs21.Click to enlarge.
For other locations in the U.S., the cost differences between these systems are quite sensitive to the relative price differences between system components. Equipment costs do not have significant geographic variation, but the wet and dry distribution systems can vary considerably. Thus, in very low-cost areas, the equipment-intensive options would become relatively more expensive (particularly systems such as radiant panels, which are unfamiliar to HVAC installers in most areas).
Integrated design can minimize construction cost premiums, as in the case of the Haverford College laboratory building, which uses ventilation air with energy recovery and local fan coils. The size of the heating and cooling plant was reduced by almost 60%; the supply air ductwork was substantially reduced, and the control system is much simpler than required by a traditional VAV system. As a result, the final construction costs for this facility were less than 90% of comparable facilities in the area [Bartholomew 2004].
All other things being equal, these alternative systems will typically use less energy than conventional VAV with reheat in laboratory applications, because they minimize simultaneous heating and cooling. Some of these systems are also inherently more efficient because they use water as a cooling medium rather than air.
It is important to note that the above discussion has focused on reheat due to internal load variation. However, in many parts of the U.S., there is also significant reheat energy use due to dehumidification requirements. The system alternatives discussed above should additionally incorporate technologies, such as energy recovery wheels or wrap-around heat-pipe coils around the cooling coils, to significantly reduce or eliminate reheat energy use needed for dehumidification.
These four alternatives should also be evaluated in terms of flexibility, maintenance, and other performance parameters. Some key points to be noted include the following:
• The ZC and FC systems are inherently modular and provide more flexibility in adding cooling capacity to the space.
• The ZC, FC, and RC systems require less space for ducts in load-driven labs, since the ducts are only for ventilation air, not thermal conditioning.
• The ZC and FC systems have distributed condensate drain pans, which need to be properly maintained and serviced. RC systems have to be carefully controlled to avoid condensation within the space.
• All these systems may have higher maintenance requirements than single-duct reheat systems, due to the greater number of components distributed throughout the building.
• The lack of familiarity of owners with these systems may be a barrier. This will require additional effort by designers to convince the owner of the benefits of these systems. Low engineering fees can also be a barrier. These higher-performance systems may require more engineering skill and time.
Continuous commissioning. Finally, it is important to note that good operations and maintenance practices can help to minimize energy use in all the system types described above. HVAC controls often deviate from design intent, which can lead to an increase in the energy use due to simultaneous heating and cooling.
Continuous commissioning and diagnostics can help to identify zones with excessive reheat, and to adjust system control and operation accordingly. A recent Labs21 best practice guide covers this topic in more detail (see www.labs21century.gov/pdf/bulletin_retrocx_508.pdf).
In conclusion, equipment load measurements from various laboratories show significant load variation between spaces. This variation can increase energy use in laboratories that have systems with reheat.
A simulation analysis showed that the magnitude of this increase varies with location, ventilation rate, and degree of variation. When designing a laboratory HVAC system, it is important to consider load variation in order to better evaluate the energy efficiency of alternative HVAC systems vis-à-vis simultaneous heating and cooling. There are several alternative system types that can minimize or even eliminate the use of reheat energy, including dual-duct-dual-fan systems, fan coil systems, zone cooling and heating coils, and radiant cooling. Continuous commissioning is also an important tool in minimizing simultaneous heating and cooling.
The authors of this document were David Frenze, PE, formerly with Earl Walls Associates (now X-nth), San Diego; Paul Mathew, Lawrence Berkeley National Laboratory (LBNL), Berkeley, Calif.; Michael Morehead, PE, Flack+Kurtz Inc., San Francisco; Dale Sartor, PE, LBNL, and William Starr Jr., Univ. of California-Davis.
Reviewers and contributors included Dan Amon, PE, U.S. Environmental Protection Agency, Washington, D.C.; Phil Bartholomew, PE, CUH2A, Princeton, N.J.; Sheila Hayter, National Renewable Energy Laboratory (NREL), Washington, D.C.; Chris Lawrence, Trox USA Inc., Alpharetta, Ga.; Will Lintner, PE, U.S. Dept. of Energy, Washington, D.C.; Peter Morris, Davis Langdon, Sacramento, Calif.; and Otto Van Geet, PE, NREL, Golden, Colo.
Production assistance for the original Best Practices Guide was provided by Jim Miller, editor, LBNL, Berkeley, and Alice Ramirez, production, LBNL, Berkeley.
U.S. Department of Energy
Energy Efficiency and Renewable Energy
Federal Energy Management Program
www.eere.energy.gov
Laboratories for the 21st Century
U.S. Environmental Protection Agency
Office of Administration and Resources Management
www.labs21century.gov
References
• Bartholomew, P., 2004. “Saving Energy in Labs,” ASHRAE Journal, February 2004. pp 35-40.
• Labs21, 2004. “Case Study: Marian E. Koshland Integrated Natural Science Center at Haverford College, Haverford, Pa.,” published by Laboratories for the 21st Century Program. Available at: www.labs21century.gov/toolkit/case_studies.htm.
htm.
• Morehead, M., 2003. “The Problem With Single-Duct VAV: The Built-in Inefficiency of a Common Lab HVAC System,” presented at the Laboratories for the 21st Century annual conference, October 2003, Denver (see www.labs21century.gov/conf/past/2003/abstracts/h1_morehead.htm
For more information
• On minimizing reheat energy use in laboratories: Paul Mathew, National Renewable Energy Laboratory, 510-486-5116, PAMathew@lbl.gov.
• On Laboratories for the 21st Century: Dan Amon, PE, U.S. Environmental Protection Agency, 202-564-7509, amon.dan@epa.gov, or Will Lintner, U.S. Dept. of Energy, Federal Energy Management Program, 202-586-3120, william.lintner@ee.doe.gov.
